Inductively coupled plasma (ICP) sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. An inductively coupled plasma source, such as the one described in U.S. Pat. No. 7,241,361, to Keller et al. for “Magnetically Enhanced, Inductively Coupled Plasma Source for a Focused Ion Beam System,” which is assigned to the assignee of the present invention, is capable of providing charged particles within a narrow energy range, thereby reducing chromatic aberrations and allowing the charged particles to be focused to a small spot.
The charged particles extracted from an inductively coupled plasma system emerge from a small (˜0.2 mm diameter) hole located in one bounding wall of the plasma reaction chamber. This particular wall is typically metal and is called the “source” electrode. However, in order not to shunt the magnetic fields around the reaction chamber, the majority of the chamber walls are made from insulating materials such as ceramic or quartz.
In current ICP ion sources, the source electrode is fixedly attached and cannot be removed after mounting. The attachment method is typically gluing (epoxy) of the source electrode into the lower opening of the source tube. In a first step, a thin metal layer is applied directly to the source tube at the locations where the source electrode will be glued. The source electrode is then glued onto the metallization layer with the assistance of a precisely positioned fixture, the glue forming the vacuum seal and none of the glue facing the plasma reaction chamber.
It has been observed that the plasma degrades the epoxy in some modes of operation, causing significant operational difficulties. One such difficulty is the plasma source being contaminated by the epoxy, leading to a redistribution of carbon from the epoxy to the reaction chamber sidewalls, shunting the inductive coupling of energy around the reaction chamber. Once a source tube has become contaminated from use, it cannot be cleaned and must be discarded, and the expense of replacing the source tube increases the cost of operation for the ICP source. Another such difficulty is due to the heating of the epoxy by the elevated source electrode temperature from plasma bombardment and eddy current heating. If the heating of the epoxy is not well compensated by active cooling, the epoxy can thermally decompose, leading to epoxy bond failure. Another such difficulty is when the vacuum seal between the source tube and the FIB column develops leaks due to openings in the epoxy seal, thereby reducing the achievable gas pressures within the source tube, leading to decreased ion generation.